The invention provides polycarbonates as a substrate material for the production of transparent injection moldings, in particular for the production of injection moldings to be coated, and moldings obtainable from the polycarbonates according to the invention. Moldings may be e.g. transparent sheets, lenses, optical storage media or carriers for optical storage media or also articles from the field of automotive glazing, such as e.g. light-diffusing panes. The invention provides in particular optical storage media or carriers for optical storage media, such as e.g. writable optical data stores, which have a good capacity for being coated and wetting capacity and are suitable e.g. for application of dyestuffs from solution, in particular from non-polar media. Furthermore, the optical injection moldings of the polycarbonates according to the invention have a relatively low tendency to soil.
Transparent injection moldings are of importance mostly in the field of glazing and of storage media.
Optical data recording materials are increasingly being used as a variable recording and/or archiving medium for large quantities of data. Examples of this type of optical data stores are CD, super-audio CD, CD-R, CD-RW, DVD, DVD-R, DVD+R, DVD-RW, DVD+RW and BD.
Transparent thermoplastics, such as, for example, polycarbonate, polymethyl methacrylate and chemical modifications thereof, are typically employed for optical storage media. Polycarbonate as a substrate material is suitable in particular for optical disks which may be written onto once and read several times and also for optical disks which may be written onto several times, and for the production of moldings from the field of automotive glazing, such as e.g. light-diffusing panes. This thermoplastic has an excellent mechanical stability, is not very susceptible to changes in dimension and is distinguished by a high transparency and impact strength.
Polycarbonate prepared by the phase interface process may be used for the production of optical data stores of the formats described above, such as e.g. for compact disks (CD) or digital versatile disks (DVD). These disks often have the property of building up a high electrical field during their production in the injection molding process. During production of the optical data store, this high field strength on the substrate leads e.g. to attraction of dust from the environment or to sticking of the injection-molded articles, such as e.g. of the disks, to one another, which reduces the quality of the finished injection-molded articles and makes the injection molding process difficult.
It is furthermore known that the electrostatic charging, in particular of disks (for optical data carriers), leads to a deficient wettability mostly with non-polar media, such as e.g. a non-polar dyestuff, or with a dyestuff application from solvents, such as e.g. dibutyl ether, ethylcyclohexane, tetrafluoropropanol, cyclohexane, methylcyclohexane or octafluoropropanol. A high electrical field on the surface of the substrate during dyestuff application to writable data stores thus causes, for example, an irregular coating with dyestuff and therefore leads to defects in the information layer.
The extent of the electrostatic charging of a substrate material may be quantified e.g. by measurement of the electrical field at a distance from its surface.
In the case of an optical data storage medium in which a dye component is applied to the surface in a spin coating process, a low absolute electrical field strength is necessary in order to obtain uniform application of the writable layer and to attain a trouble-free production process.
Furthermore, a high electrostatic field causes losses in yield in respect of the substrate material due to the facts described above. This may lead to a halt to the particular production step and is associated with high costs.
In order to solve this problem of a high static charging, several set-ups have been pursued. In general, antistatics are added to the substrate material as additives. Antistatic polycarbonate compositions are described e.g. in JP 62 207 358-A. Here, phosphoric acid derivatives, inter alia, are added to the polycarbonate as antistatics. EP 0922 728 describes various antistatics, such as polyalkylene glycol derivatives, ethoxylated sorbitan monolaurate, polysiloxane derivatives, phosphine oxides and distearylhydroxyamine, which are employed individually or as mixtures. The Japanese Application JP 62 207 358 describes esters of phosphorous acid as additives. U.S. Pat. No. 5,668,202 describes sulfonic acid derivatives. In WO 00/50 488, 3,5-di-tert-butylphenol is employed as a chain terminator in the phase interface process. This chain terminator leads to a lower static charging of the corresponding substrate material compared with conventional chain terminators. JP 62 207 358-A describes polyethylene derivatives and polypropylene derivatives as additives for polycarbonate.
However, the additives described may also have an adverse effect on the properties of the substrate material, since they tend to migrate from the material. This is indeed a desirable effect for the antistatic properties, but may lead to formation of a deposit or defective copying. Moreover, the content of oligomers in the polycarbonate may also lead to a poorer level of mechanical properties and to a lowering of the glass transition temperature. Furthermore, these additives may cause side reactions. The subsequent “end-capping” of polycarbonate which has been obtained from the transesterification process is expensive and the results achieved are not optimum. Introduction of new end groups into the material is associated with high costs.
There is thus the object of providing a composition or a substrate material which meets the requirements of a field strength on the substrate surface which is as low as possible, and avoids the disadvantages described above.